With regard to phosphate-based compounds, there are those that are amorphous and those that are crystalline with a two-dimensional layered structure or a three-dimensional network structure. Among them, crystalline phosphate-based compounds having a three-dimensional network structure are excellent in terms of heat resistance, chemical resistance, radiation resistance, low thermal expansion, etc., and their application to radioactive waste immobilization, solid electrolytes, gas adsorption/separation agents, catalysts, antimicrobial agent starting materials, low thermal expansion fillers, etc. has been considered.
Low thermal expansion fillers comprising various phosphate-based compounds have already been reported, and have been applied to sealing materials, etc. For example, Patent Document 1 discloses a sealing material comprising a mixture of a low-melting-point glass powder and a low thermal expansion material powder such as NaZr2(PO4)3, CaZr2(PO4)3, or KZr2(PO4)3, Patent Document 2 discloses NbZr2(PO4)3 powder as a filler powder for lead-free glass, and Patent Document 3 discloses Zr2(WO4)(PO4)2 powder.
Furthermore, as methods for synthesizing these zirconium phosphates, a calcination method in which starting materials are dry mixed and then calcined at a temperature of 1,000° C. or higher using a calcining furnace, etc. (e.g. Patent Document 2), a hydrothermal method in which starting materials are mixed in water or in a water-containing state, then pressurized, and heated, a wet method in which starting materials are mixed in water and then heated under normal pressure, etc. are known.